A typical cellular wireless network includes a number of base stations (BSs) each radiating to define a respective coverage area in which user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. In particular, each coverage area may operate on one or more carriers each defining a respective frequency bandwidth of coverage. In turn, each base station (BS) may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a BS and may thereby communicate via the BS with various remote network entities or with other UEs served by the BS.
Further, a cellular wireless network may operate in accordance with a particular air interface protocol (radio access technology), with communications from the BSs to UEs defining a downlink or forward link and communications from the UEs to the BSs defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Long Term Evolution (LTE) (using Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communications (GSM), IEEE 802.11 (WIFI), BLUETOOTH, among others. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between coverage areas, and other functions related to air interface communication.
In accordance with a recent version of the LTE standard of the Universal Mobile Telecommunications System (UMTS), for instance, each coverage area of a BS may operate on one or more carriers spanning 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz, with each carrier being divided primarily into subcarriers spaced apart from each other by 15 kHz. Further, the air interface is divided over time into a continuum of 10-millisecond frames, with each frame being further divided into ten 1-millisecond subframes or transmission time intervals (TTIs) that are in turn each divided into two 0.5-millisecond segments. In each 0.5 millisecond segment or in each 1 millisecond TTI, the air interface is then considered to define a number of 12-subcarrier wide “resource blocks” spanning the frequency bandwidth (i.e., as many as would fit in the given frequency bandwidth). In addition, each resource block is divided over time into symbol segments of 67 μs each, with each symbol segment spanning the 12-subcarriers of the resource block and thus supporting transmission of symbols in “resource elements.”
The LTE air interface then defines various channels made up of certain ones of these resource blocks and resource elements. For instance, on the downlink, certain resource elements across the bandwidth are reserved to define a physical downlink control channel (PDCCH) for carrying control signaling from the BS to UEs, and other resource elements are reserved to define a physical downlink shared channel (PDSCH) for carrying bearer data transmissions from the BS to UEs. Likewise, on the uplink, certain resource elements across the bandwidth are reserved to define a physical uplink control channel (PUCCH) for carrying control signaling from UEs to the BS, and other resource elements are reserved to define a physical uplink shared channel (PUSCH) for carrying bearer data transmissions from UEs to the BS.
In a system arranged as described above, when a UE enters into coverage of a BS, the UE may engage in attach signaling with the BS, by which the UE would register to be served by the BS on a particular carrier. Through the attach process and/or subsequently, the BS and supporting LTE network infrastructure may establish for the UE one or more bearers, essentially defining logical tunnels for carrying bearer data between the UE and a transport network such as the Internet.
Once attached with the BS, a UE may then operate in a “connected” mode in which the BS may schedule data communication to and from the UE on the UE's established bearer(s). In particular, when a UE has bearer data to transmit to the BS, the UE may transmit a scheduling request to the BS, and the BS may responsively allocate one or more upcoming resource blocks on the PUSCH to carry that bearer data and transmit on the PDCCH to the UE a downlink control information (DCI) message that directs the UE to transmit the bearer data in the allocated resource blocks, and the UE may then do so. Likewise, when the BS has bearer data to transmit to the UE, the BS may allocate PDSCH resource blocks to carry that bearer data and may transmit on the PDCCH to the UE a DCI message that directs the UE to receive the bearer data in the allocated resource blocks, and the BS may thus transmit the bearer data in the allocated resource blocks to the UE. LTE also supports uplink control signaling on the PUCCH using uplink control information (UCI) messages. UCI messages can carry scheduling requests from UEs, requesting the BS to allocate PUSCH resource blocks for uplink bearer data communication.
In addition, certain air interface resources are reserved for reference signaling, so that a BS can broadcast a reference signal useable by UEs for detection and evaluation of coverage, among other options. By way of example, LTE specifies, among other reference signals, a cell-specific reference signal (CRS) that UEs can detect for searching and acquiring BSs, measuring downlink quality, and estimating downlink channels for coherent demodulation and detection of downlink signals. The CRS is allocated specific resource elements in each resource block, and in conventional LTE operation referred to herein as a “legacy mode”, the CRS is transmitted on each allocated resource element in every downlink resource block during every subframe.
In this regard, the specific resource elements allocated to the CRS in each resource block—usually eight—form a pattern within the array of resource elements of each resource block, with fixed offsets of resource elements in time and subcarrier frequency. While the offset pattern is often the same across a wireless network, the pattern used by neighboring BSs on their respective downlinks may be shifted up or down relative to each other by one subcarrier frequency to avoid interference that would otherwise occur from the CRS being transmitted on the same subcarrier frequency at the same time.
Unfortunately, however, such shifting of the pattern may lead to other interference problems. Specifically, the resource elements carrying a CRS in downlink resource blocks transmitted from one BS may interfere with resource elements carrying bearer data in downlink resource blocks transmitted from a neighboring BS. Consequently, downlink data rates may decrease and overall network performance may suffer.